Primarily a matter of timing
There aren't any "back rooms" available in many cases, unless you break into the school kitchen, and even if there were, how would anyone manipulate dozens and dozens of precincts? I suppose there could be monkey business at LD and county conventions, at least in theory, but by that time the nominee is always known anyhow. Plus I defy anyone to break a rule at a Democratic convention: you'll have 15 parliamentary experts shouting "point of order" faster than you can withdraw the motion.
The "back room" stuff is a lot of garbage thrown out there by people who don't know what they are talking about. Anyone who has ever helped organize caucuses and conventions knows that.
I'll grant that a primary will draw more people, but that doesn't mean it would necessarily have a bigger impact on the selection of Democratic delegates. And of course there is the age-old problem that DNC rules requiring "known Democrats" to choose delegates conflicts with our state's tradition of not having party registration, leading to the filling out of "declarations" that can rub people the wrong way.
But the 2004 caucuses were extremely well attended and brought many people into the process.
It's understandable the some folks may not be comfortable going to a caucus, but the flip side is that it's the only time most people are asked to sit down, discuss things, argue, and then vote. Since most people avoid the off-year caucuses, once every four years doesn't seem all that unreasonable of a requirement. Some people find, much to their surprise, that they really like meeting neighbors with whom they share similar political views.
More important is the timing. With California moving its primary to Feb. 5th, it would seem that if Washington wants in on the action it better follow suit. According to the P-I, talks will continue on that issue.